The 3′,5′-cyclic nucleotide phosphodiesterases (PDEs) comprise a large class of enzymes divided into at least eleven different families which are structurally, biochemically and pharmacologically distinct from one another. The enzymes within each family are commonly referred to as isoenzymes, or isozymes. A total of more than fifteen gene products is included within this class, and further diversity results from differential splicing and post-translational processing of those gene products. The present invention is primarily concerned with the four gene products of the fourth family of PDEs, i.e., PDE4A, PDE4B, PDE4C, and PDE4D. These enzymes are collectively referred to as being isoforms or subtypes of the PDE4 isozyme family. Further below will be found a more detailed discussion of the genomic organization, molecular structure and enzymatic activity, differential splicing, transcriptional regulation and phosphorylation, distribution and expression, and selective inhibition of the PDE4 isozyme subtypes.
The PDE4s are characterized by selective, high affinity hydrolytic degradation of the second messenger cyclic nucleotide, adenosine 3′,5′-cyclic monophosphate (cAMP), and by sensitivity to inhibition by rolipram. A number of selective inhibitors of the PDE4s have been discovered in recent years, and beneficial pharmacological effects resulting from that inhibition have been shown in a variety of disease models. See, e.g., Torphy et al., Environ. Health Perspect. 102 Suppl. 10, 79-84, 1994; Duplantier et al., J. Med. Chem. 39 120-125, 1996; Schneider et al., Pharmacol. Biochem. Behav. 50 211-217, 1995; Banner and Page, Br. J. Pharmacol. 114 93-98, 1995; Barnette et al., J. Pharmacol. Exp. Ther. 273 674-679, 1995; Wright et al. “Differential in vivo and in vitro bronchorelaxant activities of CP-80633, a selective phosphodiesterase 4 inhibitor,” Can. J. Physiol. Pharmacol. 75 1001-1008, 1997; Manabe et al. “Anti-inflammatory and bronchodilator properties of KF19514, a phosphodiesterase 4 and 1 inhibitor,” Eur. J. Pharmacol. 332 97-107, 1997; and Ukita et al. “Novel, potent, and selective phosphodiesterase-4 inhibitors as antiasthmatic agents: synthesis and biological activities of a series of 1-pyridylnaphthalene derivatives,” J. Med. Chem. 42 1088-1099, 1999. Accordingly, there continues to be considerable interest in the art with regard to the discovery of further selective inhibitors of PDE4s.
The present invention is also concerned with the use of selective PDE4 inhibitors for the improved therapeutic treatment of a number of inflammatory, respiratory and allergic diseases and conditions, but especially for the treatment of asthma; chronic obstructive pulmonary disease (COPD) including chronic bronchitis, emphysema, and bronchiectasis; chronic rhinitis; and chronic sinusitis. Heretofore in the art, however, the first-line therapy for treatment of asthma and other obstructive airway diseases has been the nonselective PDE inhibitor theophylline, as well as pentoxifylline and IBMX, which may be represented by Formulas (0.0.1), (0.0.2), and (0.0.3), respectively: 
Theophylline, which has the PDEs as one of its biochemical targets, in addition to its well characterized bronchodilatory activity, affects the vasculature of patients with increased pulmonary artery pressure, suppresses inflammatory cell responses, and induces apoptosis of eosinophils. Theophylline's adverse events, most commonly cardiac dysrhythmias and nausea, are also mediated by PDE inhibition, however, leading to the search for more selective inhibitors of PDEs that are able to suppress both immune cell functions in vitro and allergic pulmonary inflammation in vivo, while at the same time having improved side-effect profiles. Within the airways of patients suffering from asthma and other obstructive airway diseases, PDE4 is the most important of the PDE isozymes as a target for drug discovery because of its distribution in airway smooth muscle and inflammatory cells. Several PDE4 inhibitors introduced to the art thus far have been designed to have an improved therapeutic index concerning the cardiovascular, gastrointestinal, and central nervous system side effects of the above-mentioned nonselective xanthines.
Airflow obstruction and airway inflammation are features of asthma as well as COPD. While bronchial asthma is predominantly characterized by an eosinophilic inflammation, neutrophils appear to play a major role in the pathogenesis of COPD. Thus, PDEs that are involved in smooth muscle relaxation and are also found in eosinophils as well as neutrophils probably constitute an essential element of the progress of both diseases. The PDEs involved include PDE3s as well as PDE4s, and bronchodilating inhibitors have been discovered which are selective PDE3 inhibitors and dual PDE3/4 selective inhibitors. Examples of these are milrinone, a selective PDE3 inhibitor, as well as zardaverine and benafentrine, both dual PDE3/4 selective inhibitors, which may be represented by Formulas (0.0.4), (0.0.5), and (0.0.6), respectively: 
However, benafentrine results in bronchodilation only when administered by inhalation, and zardaverine produces only a modest and short-lived bronchodilation. Milrinone, a cardiotonic agent, induces short-lived bronchodilation and a slight degree of protection against induced bronchoconstriction, but has marked adverse events, e.g., tachycardia and hypotension. Unsatisfactory results have also been obtained with a weakly selective PDE4 inhibitor, tibenelast, and a selective PDE5 inhibitor, zaprinast, which may be represented by Formulas (0.0.7) and (0.0.8): More relative success has been obtained in the art with the discovery and development of selective PDE4 inhibitors.
In vivo, PDE4 inhibitors reduce the influx of eosinophils to the lungs of allergen-challenged animals while also reducing the bronchoconstriction and elevated bronchial responsiveness occurring after allergen challenge. PDE4 inhibitors also suppress the activity of immune cells, including CD4+ T-lymphocytes, monocytes, mast cells, and basophils; reduce pulmonary edema; inhibit excitatory nonadrenergic noncholinergic neurotransmission (eNANC); potentiate inhibitory nonadrenergic noncholinergic neurotransmission (iNANC); reduce airway smooth muscle mitogenesis; and induce bronchodilation. PDE4 inhibitors also suppress the activity of a number of inflammatory cells associated with the pathophysiology of COPD, including monocytes/macrophages, CD8+ T-lymphocytes, and neutrophils. PDE4 inhibitors also reduce vascular smooth muscle mitogenesis and, and potentially interfere with the ability of airway epithelial cells to generate pro-inflammatory mediators. Through the release of neutral proteases and acid hydrolases from their granules, and the generation of reactive oxygen species, neutrophils contribute to the tissue destruction associated with chronic inflammation, and are further implicated in the pathology of conditions such as emphysema.
Selective PDE4 inhibitors which have been discovered thus far that provide therapeutic advantages include SB-207,499, identified as ARIFLO®, which may be represented by Formula (0.1.9): SB-207,499, administered orally at dosages of 5, 10, and 15 mg b.i.d., has produced significant increases in trough FEV1 (forced expiratory volume in 1 second) from placebo at week 2 of a study involving a large number of patients. Another potent, selective PDE4 inhibitor, CDP840, has shown suppression of late reactions to inhaled allergen after 9.5 days of oral administration at doses of 15 and 30 mg in a group of patients with bronchial asthma. CDP840 may be represented by Formula (0.0.9): 
PDEs have also been investigated as potential therapy for obstructive lung disease, including COPD. In a large study of SB-207,499 in patients with COPD, the group of patients receiving 15 mg b.i.d. has experienced a progressive improvement in trough FEV1, reaching a maximum mean difference compared with placebo of 160 mL at week 6, which represents an 11% improvement. See Compton et al., “The efficacy of Ariflo (SB207499), a second generation, oral PDE4 inhibitor, in patients with COPD,” Am. J. Respir. Crit. Care Med. 159, 1999. Patients with severe COPD have been observed to have pulmonary hypertension, and decreases in mean pulmonary artery pressure under clinical conditions have been achieved by oral administration of the selective PDE3 inhibitors milrinone and enoximone. Enoximone has also been shown to reduce airway resistance in patients hospitalized with decompensated COPD. See Leeman et al., Chest 91 662-6, 1987. Using selective PDE3 inhibition by motapizone and selective PDE5 inhibition by zaprinast, it has been shown that combined inhibition of PDE 3 and 5 exerts a relaxation of pulmonary artery rings which corresponds broadly to the pattern of PDE isozymes found in the pulmonary artery smooth muscle. See Rabe et al., Am. J. Physiol. 266 (LCMP 10): L536-L543, 1994. The structures of milrinone and zaprinast are shown above as Formulas (0.0.4) and (0.0.8), respectively. The structures of enoximone and motapizone may be represented by Formulas (0.0.10) and (0.0.11), respectively: 
The effects of PDE4 inhibitors on various inflammatory cell responses can be used as a basis for profiling and selecting inhibitors for further study. These effects include elevation of cAMP and inhibition of superoxide production, degranulation, chemotaxis, and tumor necrosis factor alpha (TNFα) release in eosinophils, neutrophils and monocytes. PDE4 inhibitors may induce emesis, i.e., nausea and vomiting, which, as expected, is an adverse effect. The emesis adverse effect became apparent when PDE4 inhibitors were first investigated for CNS indications such as depression, when rolipram and denbufylline were used in clinical trials. Rolipram and denbufylline may be represented by Formulas (0.0.12) and (0.0.13), respectively: 
The mechanism(s) by which PDE4 inhibitors may potentially induce emesis is/are uncertain, but a study of the PDE4 inhibitor Ro-20-1724 suggests that nausea and vomiting are at least partially mediated by the emesis centers in the brain. Gastrointestinal adverse events may be caused by local effects, e.g., rolipram is a very potent stimulator of acid secretion from gastric parietal cells, and the resulting excess acid, by producing local irritation, may exacerbate gastrointestinal disturbances. Ro-20-1724 may be represented by Formula (0.0.14): Efforts to minimize or eliminate the above-mentioned adverse events sometimes associated with PDE4 inhibitors have included creating inhibitors which do not penetrate the central nervous system, and administering PDE4 inhibitors by inhalation rather than orally.
With regard to the PDE4 subtypes, A, B, C, and D, it has been found that PDE4C is usually less sensitive to all inhibitors; whereas, with respect to the subtypes A, B, and D, there is as yet no clear evidence of inhibitor specificity, which is defined as a 10-fold difference in IC50 values. While most inhibitors, especially RS-25,344, are more potent against PDE4D, this does not amount to selectivity. RS-25,344 may be represented by Formula (0.0.15): On the other hand, there is a stereoselective effect on the elevation of cAMP in a range of cell types, which has been demonstrated with the results of an investigation of CDP840, shown above as Formula (0.0.9), and its less active enantiomer CT-1731, which is represented by Formula (0.0.16): 
It has been known for some time that rolipram had the ability to interact with a high-affinity binding site on brain membranes, and it was later established in the art that this high-affinity rolipram binding site (Sr), which is distinct from the catalytic site (Sc), exists in a truncated recombinant PDE4A and a full-length recombinant PDE4B. More recently, Sr has been identified on all four PDE4 subtypes. See Hughes et al., Drug Discovery Today 2(3) 89-101, 1997. The presence of Sr appears to have a profound effect on the ability of certain inhibitors such as rolipram and RS-25,344 to inhibit the catalytic activity of PDE4 isozymes.
The impact of residues on inhibitor binding is also significant. A single amino acid substitution (alanine for aspartate) in the catalytic region of PDE4B has been shown to be critical for inhibition by rolipram, and this appears to be a class effect because related inhibitors RP-73,401 and Ro-20-1724 also lose potency on the mutant enzyme. However, the role of binding of inhibitors to the Sc or to the Sr, in terms of elevation of cAMP and inhibition of cell responses, is not fully understood at the present time.
RP-73,401, in guinea-pig studies, has been found to be active in (1) the inhibition of antigen-induced lung eosinophilia and eosinophil peroxidase (EPO), Banner, K. H., “The effect of selective phosphodiesterase inhibitors in comparison with other anti-asthma drugs on allergen-induced eosinophilia in guinea-pig airways,” Pulm. Pharmacol. 8 37-42, 1995; (2) antigen-induced bronchoalveolar lavage (BAL) eosinophilia, Raeburn et al., “Anti-inflammatory and bronchodilator properties of RP73401, a novel and selective phosphodiesterase Type IV inhibitor,” Br. J. Pharmacol. 113 1423-1431, 1994; (3) antigen-induced airway eosinophilia and platelet activating factor-(PAF)- and ozone-induced airway hyper-responsiveness (AHR), Karlsson et al., “Anti-inflammatory effects of the novel phosphodiesterase IV inhibitor RP73401,” Int. Arch. Allergy Immunol. 107 425-426, 1995; and (4) IL-5 induced pleural eosinophila. Development of RP-73,401, piclamilast, has been discontinued. Piclamilast may be represented by Formula (0.0.17): 
A related series of compounds is represented by RPR-132294 and RPR-132703, which have been demonstrated in rat studies to have activity in the inhibition of antigen-induced bronchospasm; Escott et al., “Pharmacological profiling of phosphodiesterase 4 (PDE4) inhibitors and analysis of the therapeutic ratio in rats and dogs,” Br. J. Pharmacol. 123(Proc. Suppl.) 40P, 1998; and Thurairatnam et al., “Biological activity and side effect profile of RPR-132294 and RPR-132703—novel PDE4 inhibitors,” XVth EFMC Int. Symp. Med. Chem., 1998. The structure of RPR-132294 may be represented by Formula (0.0.18): 
Another compound whose development has been discontinued is WAY-PDA-641, filaminast, which in studies in the dog, has been found to be active in the inhibition of seratonin-induced bronchoconstriction. Filaminast may be represented by Formula (0.0.19): 
It has been suggested in the art that PDE4 inhibitors that have a high affinity at the Sr can be correlated with emesis and increased gastric acid secretion. RS-23,544, RP-73,401, and CP-80,633 elicit emesis and have a high affinity at the Sr. CDP840 and SB-207,499 have a comparatively low affinity at the Sr, but CDP840 has a significantly higher potency at the Sc than does SB-207,499. CDP840 has been demonstrated to provide significant inhibition of late-phase response in the treatment of asthma without any adverse events of nausea or headache. Another PDE4 inhibitor that has been shown to have adverse events of nausea and vomiting is BRL-61,063, also referred to as cipamfylline, which is described further below. The development of CDP840 has been discontinued, while CP-80,633, atizoram, has been advanced into clinical studies. CP-80,633 and BRL-61,063 may be represented by Formulas (0.0.20) and (0.1.12), respectively: 
Another compound which is in development is LAS-31025, arofylline, which in guinea-pig studies, has been found to be active in the inhibition of antigen-induced bronchoconstriction; Beleta, B. J., “Characterization of LAS31025: a new selective PDE IV inhibitor for bronchial asthma,” Third Int. Conf. On Cyclic Nucleotide Phosphodiesterase: From Genes to Therapies, Glasgow, UK, 1996, Abstract 73. LAS-31025, arofylline, may be represented by Formula (0.0.21): 
A number of PDE4 inhibitors have been advanced in development. For example, the effects of V-11294A on LPS-stimulated ex vivo TNF release and PHA induced lymphocyte proliferation have been determined in a randomized, double-blind placebo-controlled study which has found that an oral dose of 300 mg is effective in reducing TNF levels and lymphocyte proliferation; Landells et al., “Oral administration of the phosphodiesterase (PDE) 4 inhibitor, V11294A inhibits ex-vivo agonist-induced cell activation,” Eur. Resp. J. 12(Suppl. 28) 362s, 1998; and Gale et al., “Pharmacodynamic-pharmacokinetic (PD/PK) profile of the phosphodiesterase (PDE) 4 inhibitor, V 1294A, in human volunteers,” Am. J. Respir. Crit. Care Med. 159 A611, 1999.
The compound D4418 has been administered to healthy volunteers in a single escalating dose, randomized, placebo-controlled Phase I study; Montana et al., “Activity of D4418, a novel phosphodiesterase 4 (PDE4) inhibitor, effects in cellular and animal models of asthma and early clinical studies,” Am. J. Respir. Crit. Care Med. 159 A108, 1999. D4418 is a moderately potent PDE4 inhibitor with an IC50 of 200 nM. It has good oral absorption; a 200 mg dose provides a plasma Cmax of 1.4 μg/ml. D4418 has been discontinued from development due to its moderate potency, and has been replaced by the preclinical development candidate D4396.
V-11294A and D4418 may be represented by Formulas (0.0.22) and (0.0.23), respectively: 
Another compound, CI-1018, has been evaluated in 54 subjects and no adverse events were reported at doses up to 400 mg; Pruniaux et al., “The novel phosphodiesterase inhibitor CI-1018 inhibits antigen-induced lung eosinophilia in sensitized brown-norway rats—comparison with rolipram,” Inflammation S-04-6, 1999. CI-1018 has been demonstrated to have good oral bioavailability (57% in the rat) and good oral potency of with an ED50 of 5 mg/kg in that same species. CI-1018 is a relatively weak PDE4 inhibitor with an IC50 of 1.1 μM in U937 cells. CI-1018 has also been identified as, or associated with as closely related in structure to, PD-168787, which in rat studies has been demonstrated to have activity in the inhibition of antigen-induced eosinophilia; Pascal et al., “Synthesis and structure-activity relationships of 4-oxo-1-phenyl-3,4,6,7-tetrahydro-[1,4]-diazepino[6,7,1-hi]indolines: novel PDE4 inhibitors,” 215th ACS, Dallas, USA, MEDI 50, 1998. Inferred structures for CI-1018 and PD-168787 belong to a diazepinone class whose nucleus may be represented by Formula (0.0.24): 
The above-mentioned compounds have also been evaluated in animal models which demonstrate their PDE4 inhibition activity. For example, V-11294A, in guinea-pig studies, has been found to be active in the inhibition of antigen-induced bronchoconstriction; Cavalla et al., “Activity of V 11294A, a novel phosphodiesterase 4 (PDE4) inhibitor, in cellular and animal models of asthma,” Amer. J. Respir. Crit. Care Med, 155 A660, 1997. D4418, in guinea-pig studies, has been found to be active in the inhibition of antigen-induced early and late phase bronchoconstriction and BAL eosinophilia; Montana, et al., Ibid. CI-1018, in rat studies, has been found to be active in the inhibition of antigen-induced eosinophilia; Burnouf, et al., “Pharmacology of the novel phosphodiesterase Type 4 inhibitor, CI-1018,” 215th ACS Nat. Meeting, MEDI 008, 1998.
Other compounds which have been advanced in development include CDC-3052, D-22888, YM-58997, and roflumilast, which may be represented by Formulas (0.0.27), (0.0.28), (0.0.29), and (0.0.30), respectively: 
CDC-3052 has been discontinued from development, but has been succeeded by very potent inhibitors of PDE4 such as the compound represented by Formula (0.0.31), and by the anti-inflammatory compound CDC-801 represented by Formula (0.0.32), respectively: The compound of Formula (0.0.32) is reported to have IC50 values of 42 pM and 130 nM as an inhibitor of PDE4 and TNF production, respectively; Muller et al., “N-Phthaloyl beta-aryl-beta-amino derivatives: Potent TNF-alpha and PDE4 inhibitors,” 217th American Chemical Society, Annheim, Germany, MEDI 200, 1999; and Muller et al., “Thalidomide analogs and PDE4 inhibition,” Bioorg. Med. Chem. Letts. 8 2669-2674, 1998.
CDC-801 is from a series of compounds based on thalidomide and has been developed primarily to improve the TNF-α inhibitory activity of thalidomide for the treatment of autoimmune diseases. Thalidomide may be represented by Formula (0.0.33): CDC-801 has also been studied for the treatment of Crohn's disease, a chronic granulomatous inflammatory disease of unknown etiology commonly involving the terminal ileum, with scarring and thickening of the bowel wall which frequently leads to intestinal obstruction and fistula and abscess formation. Crohn's disease has a high rate of recurrence after treatment.
YM-58997 has an IC50 value of 1.2 nM against PDE4; Takayama et al., “Synthetic studies on selective Type IV phosphodiesterase (PDE IV) inhibitors,” 214th American Chemical Society, Las Vegas, USA, MEDI 245, 1997. YM-58997 has a 1,8-naphthyridin-2-one structure, as does YM-976.
Roflumilast has been studied for the treatment of both COPD and asthma, and has an IC50 value of 3.5 nM in standard in vitro guinea-pig models of asthma. The use of roflumilast and a surfactant for the treatment of adult respiratory distress syndrome (ARDS) has also been described.
AWD-12,281, which is now designated as loteprednol, has been shown to be active in a rat model of allergic rhinitis, as described further below in a section which deals with allergic rhinitis and the use of PDE4 inhibitors to treat it. AWD-12,281 may be represented by Formula (0.0.34): 
Compounds related in structure to CDP840, shown further above as Formula (0.0.9), include L-826,141, which has been reported to have activity in a rat model of bronchitis; Gordon et al., “Anti-inflammatory effects of a PDE4 inhibitor in a rat model of chronic bronchitis,” Am. J. Respir. Crit. Care Med. 159 A33, 1999. Another such compound is related in structure to those reported in Perrier et al., “Substituted furans as inhibitors of the PDE4 enzyme,” Bioorg. Med. Chem. Letts. 9 323-326, 1999, and is represented by Formula (0.0.35): 
Other compounds which been found to be very potent PDE4 inhibitors are those represented by Formulas (0.0.36), (0.0.37), and (0.0.38): 
Compounds have been created which combine PDE4 and matrix metalloproteinase (MMP) inhibitory activity in a single molecule; Groneberg et al., “Dual inhibition of phosphodiesterase 4 and matrix metalloproteinases by an (arylsulfonyl)hydroxamic acid template,” J. Med. Chem. 42(4) 541-544, 1999. Two examples of such compounds are represented by Formulas (0.0.39) and (0.0.40): The respective IC50 values for the compounds of Formulas (0.1.36) and (0.1.37) using a guinea-pig macrophage PDE4 assay were 1 nM and 30 nM.
The compounds identified as KF19514 and KF17625 have been shown in guinea-pig studies to have activity in the inhibition of the following: histamine-induced and antigen-induced bronchoconstriction; PAF-induced lung eosinophilia and antigen-induced BAL eosinophilia; acetylcholine (ACh)-induced AHR; PAF-induced BAL eosinophilia and neutrophilia, and AHR; antigen-induced bronchospasm; and anaphylactic bronchoconstriction; Fujimura et al., “Bronchoprotective effects of KF-19514 and cilostazol in guinea-pigs in vivo,” Eur. J. Pharmacol. 327 57-63, 1997; Manabe et al., Ibid.; Manabe et al., “KF19514, a phosphodiesterase 4 and 1 inhibitor, inhibits PAF-induced lung inflammatory responses by inhaled administration in guinea-pigs,” Int. Arch. Allergy Immunol. 114 389-399, 1997; Suzuki et al., “New bronchodilators. 3. Imidazo[4,5-c][1,8]naphthyridin-4(5H)-ones,” J. Med. Chem. 35 4866-4874, 1992; Matsuura et al., “Substituted 1,8-naphthyridin-2(1H)-ones as selective phosphodiesterase IV inhibitors,” Biol. Pharm. Bull. 17(4) 498-503, 1994; and Manabe et al., “Pharmacological properties of a new bronchodilator, KF17625,” Jpn. J. Pharmacol. 58(Suppl. 1) 238P, 1992. KF19514 and KF17625 may be represented by Formulas (0.0.41) and (0.0.42): 
The reported potency and lack of emesis in a series of indandiones suggests that the hypothesis that has related side-effects such as emesis to the ratio of affinity for the PDE4 enzyme relative to that for the high affinity rolipram binding site (HARBS) is erroneous. Such indandiones may be represented by Formulas (0.0.43) and (0.0.44): 
The PDE4 inhibitors that have been created heretofore fall into a significant number of different classes in terms of their chemical structures. Such classes have been as diverse as phenanthridines and naphthyridines. One class of PDE4 inhibitors are lignans such as T-440, which has been demonstrated to have activity in the inhibition of the following: early phase bronchoconstriction induced by antigen, histamine, LTD4, U-46619, Ach, neurokinin A, and endothelin-1; allergen-induced early phase and late phase bronchoconstriction and BAL eosinophilia; and ozone-induced AHR and airway epithelial injury. Optimization of the PDE4 inhibitory potency of such compounds has led to the discovery of T-2585, one of the most potent PDE4 inhibitors described to date with an IC50 value of 0.13 nM against guinea-pig lung PDE4. T-440 and T-2585 may be represented by Formulas (0.0.45) and (0.0.46): 
Another class of PDE4 inhibitors consists of benzofurans and benzothiophenes. In particular, furan and chroman rings have been utilized as surrogates for the cyclopentylether of the rolipram pharmacophore. An example of such a compound is one that is apparently related in structure to BAY 19-8004, and which may be represented by Formula (0.0.47): Another benzofuran-type compound has been reported to have an IC50 value of 2.5 nM, and may be represented by Formula (0.0.48): 
A compound with a related structure, which is not, however, a benzofuran, is characterized by a fused dioxicin ring and is reported to produce almost complete inhibition of canine tracheal PDE4 at 100 nM. This compound may be represented by Formula (0.0.49): 
Quinolines and quinolones are a further class of PDE4 inhibitor structures, and they serve as surrogates for the catechol moiety of rolipram. This compound and two compounds of similar structure may be represented by Formulas (0.0.50), (0.0.51), and (0.0.52): 
Purines, xanthines, and pteridines represent yet further classes of chemical compounds to which PDE4 inhibitors described heretofore in the art belong. The compound V-11294A described further above and represented by Formula (0.0.22), is a purine. A PDE4 inhibitor which is a xanthine compound, the class of compounds to which theophylline belongs, has been described in the art; Montana et al., “PDE4 inhibitors, new xanthine analogues,” Bioorg. Med. Chem. Letts. 8 2925-2930, 1998. The xanthine compound may be represented by Formula (0.0.54): 
A potent PDE4 inhibitor belonging to the pteridine class of compounds has been demonstrated to have an IC50 value of 16 nM against a PDE4 derived from tumor cells and to inhibit the growth of tumor cells at micromolar concentrations; Merz et al., “Synthesis of 7-Benzylamino-6-chloro-2-piperazino-4-pyrrolidinopteridine and novel derivatives free of positional isomers. Potent inhibitors of cAMP-specific phosphodiesterase and of malignant tumor cell growth,” J. Med. Chem. 41(24) 4733-4743, 1998. The pteridine PDE4 inhibitor may be represented by Formula (0.0.55): 
Triazines represent a still further class of chemical compounds to which PDE4 inhibitors belong that have been described in the art heretofore. Two such triazines have been described which display bronchodilator activity and are potent relaxant agents in a guinea-pig trachea model. These compounds, which may be represented by Formulas (0.0.56) and (0.0.57) below, are also moderately potent PDE4 inhibitors with IC50 values of 150 and 140 nM, respectively: A triazine having a structure assumed to be closely related to that of the compounds of Formulas (0.0.56) and (0.0.57) is UCB-29936, which has been demonstrated to have activity in a murine model of septic shock; Danhaive et al., “UCB29936, a selective phosphodiesterase Type IV inhibitor: therapeutic potential in endotoxic shock,” Am. J. Respir. Crit. Care. Med. 159 A611, 1999.
Efforts have also been made in the art to improve the selectivity of PDE4 inhibitors with respect to the A through D subtypes described further above. There are presently four known isoforms (subtypes) of the PDE4 isozyme, encompassing seven splice variants, also described further above. The PDE4D isoform mRNA is expressed in inflammatory cells such as neutrophils and eosinophils, and it has been suggested in the art that D-selective inhibitors of PDE4 will provide good clinical efficacy with reduced side-effects. A nicotinamide derivative displaying selectivity for inhibition of the PDE4D isoform has been described; WO 98/45268; as well as a naphthyridine derivative reported to be a PDE4D selective inhibitor; WO 98/18796. These compounds may be represented by Formulas (0.0.58) and (0.0.59), respectively: 
Another nicotinamide compound has been described in the art which may be useful in the treatment of CNS diseases such as multiple sclerosis; GB-2327675; and a rolipram derivative has been described in the art which is a PDE4 inhibitor which binds with equal affinity to both the catalytic and the HARB sites on human PDE4B2B; Tian et al., “Dual inhibition of human Type 4 phosphodiesterase isostates by (R,R)-(+/−)-methyl-3-acetyl-4-[3-(cyclopentyloxy)-4-methoxyphenyl]-3-methyl-1-pyrrolidine carboxylate,” Biochemistry 37(19) 6894-6904, 1998. The nicotinamide derivative and the rolipram derivative may be represented by Formulas (0.0.60) and (0.0.61), respectively: 
Further background information concerning selective PDE4 isozymes may be found in publications available in the art, e.g., Norman, “PDE4 inhibitors 1999,” Exp. Opin. Ther. Patents 9(8) 1101-1118, 1999 (Ashley Publications Ltd.); and Dyke and Montana, “The therapeutic potential of PDE4 inhibitors,” Exp. Opin. Invest. Drugs 8(9) 1301-1325, 1999 (Ashley Publications Ltd.).